The prior art for treatment of distillation bottom streams from isocyanate preparation that comprise the distillation residues mentioned describes various processes. General aims of what is called “residue treatment” are the maximization of the isocyanate yield, the minimization of the amount of residue obtained that has been largely to entirely freed of isocyanate, operationally reliable configuration of the residue treatment and the best possible inexpensive and simple utilization of the residue volume no longer utilizable for the isocyanate preparation process.
The following processes are known in principle:
In principle, a distillate bottom stream comprising the distillation residue can be combusted continuously or batchwise. The process is technically simple and can be used to raise useful steam if a plant for thermal utilization which is suitable for the purpose exists in the vicinity of the isocyanate production plant, in order to assure disposal via a pipeline connection. However, the great disadvantage of this process is the loss of yield, caused by the fact that the distillation bottom stream comprising the distillation residue always also contains proportions of the isocyanate product of value which is burnt as well. If the distillation of the isocyanate were to be conducted in such a way that the isocyanate were to be removed completely or virtually completely from the distillation bottoms, what would remain would be a solid distillation bottoms phase that could be processed only with great difficulty and consisted almost exclusively of residue. In order to avoid this, distillation conditions are typically chosen such that the bottom product from the distillation column remains liquid, but this is only possible if it still contains a substantial proportion of the isocyanate to be prepared, which is thus inevitably sent to the combustion as well.
To minimize the isocyanate yield losses, the distillation bottom stream can be transferred into a stirred and heated vessel and mixed with high-boiling hydrocarbons, preferably bitumen, that are inert under the distillation conditions to distill off a maximum amount of the free isocyanate still present in the residue (EP 0 548 685 A2). The remaining residue that has been largely freed of isocyanate can be discharged as a free-flowing solid and sent to incineration. Disadvantages of this process include not only the use of a substance foreign to the process (bitumen) but also yield losses through polymerization of the isocyanate since the process includes high residence times at high temperature.
A further process for residue treatment, described in EP 0 626 368 A1, is characterized by the use of heated, product-agitating vacuum driers with a horizontal shaft. The use of bitumen, for example, has the effect that, as in the abovementioned example of the process according to EP 0 548 685 A2, the remaining residue is obtained as a free-flowing solid that can be used as a fuel in cement works for example. The advantage of this process over that mentioned above is an increase in yield. However, the use of mechanical moving parts harbors the risk of elevated wear and associated maintenance expenditure. This is especially true when the residue to be worked up has a very high viscosity. The use of drying apparatuses of this kind with mechanical moving parts, even though it is basically tried and tested, is therefore not always free of challenges in everyday operation. The present invention is concerned with such challenges inter alia.
The patent literature also describes processes in which isocyanate distillation bottom streams comprising distillation residues are chemically converted in order to obtain industrially utilizable materials of value, for example the reaction of a distillation bottom stream comprising residue from the preparation of tolylene diisocyanate with alkanolamine (U.S. Pat. No. 5,902,459) or with isocyanates of the diphenylmethane series (DE 42 11 774 A1, U.S. Pat. No. 3,694,323).
The hydrolysis of isocyanate distillation bottom streams with water to achieve recovery of the starting amine, especially in the preparation of tolylene diisocyanate (TDI hereinafter), is a field that has been worked on for some long time already and is described in U.S. Pat. Nos. 3,128,310, 3,331,876, GB 795,639, DE 27 03 313 A1 and EP 1 935 877 A1 for example. What is unsatisfactory in these processes is that a portion of the isocyanate product of value has to be hydrolyzed back to the starting material and phosgenated again. This does send the isocyanate present in the distillation bottom stream to a viable physical use, but it would be desirable to be able to recover the isocyanate as such from the distillation bottom streams.
EP 1 413 571 A1 and EP 1 371 633 A1 are concerned with the optimization of the workup of TDI by using a dividing-wall column in the distillation which results, inter alia, in a reduction in the TDI content in the bottom product. But here too, it is not possible to completely prevent an isocyanate-containing distillation bottom stream from being obtained.
EP 0 017 972 A1 describes a process for separating TDI and/or higher-boiling solvents from distillation bottom streams that arise in the preparation of TDI by phosgenation of tolylenediamine by evaporative concentration in a fluidized bed at temperatures of 140 to 280° C. In this process, the droplets of the distillation bottom stream that have been introduced into the fluidized bed vessel by means of the introduction device are sprayed onto the surface of the initially charged particles, where they spread, which leads to evaporation of the product of value (TDI and/or solvent) and to formation of dish-shaped pellets of residue that are free of material of value. Such a granulation process generally does not have cycle times of any great length and has to be run down for the purpose of intermediate cleaning after particular time intervals. This is disadvantageous for the present case of workup of isocyanate-containing distillation bottom streams owing to the required inertization of the reaction space and the high temperatures, and the running-up problems.
WO 2014/009342 A1 is concerned with a spray drying process for obtaining monomeric isocyanate (i.e. the isocyanate to be prepared, which is desired as the target product, as opposed to unwanted high molecular weight polymers containing isocyanate groups) from distillation residue-containing bottom streams. The spray drying described affords a dried residue that has been largely to completely freed of monomeric isocyanate and a stream comprising monomeric isocyanate. Performance of this process requires a special reactor, and so it generally cannot be integrated into an existing isocyanate production plant without major refitting.
Further improvements in the field of workup of isocyanate distillation bottom streams comprising not only distillation residues but also proportions of the isocyanate to be prepared are therefore desirable. More particularly, it would be desirable to separate the desired target product, i.e. the isocyanate to be prepared, from the distillation bottom stream in a simple, operationally stable and economic manner in order to minimize yield losses and maintenance expenditure in continuous operation on the industrial scale as well, specifically also when the composition of the distillation bottom stream containing the residue is subject to production-related variations. It would also be desirable if such an improved process could be integrated in a simple manner into an already existing process that uses prior art drying apparatus (e.g. the heated, product-agitating vacuum driers with a horizontal shaft that have been mentioned).